In any network in which devices share a single transmission channel (e.g., a wireless network, a shared bus network, etc.), every device can hear all or most of the other devices. In such a network when two devices transmit a signal at the same time, the two transmitted signals are considered to have collided. It's typically desirable to avoid such collisions, since it often will result in both signals being unintelligible to their target devices, causing no data to be sent and thus losing use of the transmission medium (i.e., the transmission channel) for a time. Both signals will then have to be resent, lowering the data throughput of the network. If this happens often enough, the entire transmission medium could be rendered ineffective.
As a result, in order to reduce the number of collisions and maximize data throughput, it is generally desirable to provide some kind of protocol to minimize the chance that two devices will transmit a signal at the same time. Several different protocols are available.
A frequency division multiple access (FDMA) divides the transmission medium up into discrete frequency ranges and assigns each device or network to a given range. A device or network can only transmit in its assigned frequency range, but knows that it won't have a collision with another device or network. Broadcast radio and television use an FDMA protocol. In may cases, this protocol is layered on top of another protocol, with FDMA assigning a frequency range to a network, and some other protocol coordinating network transmissions within the assigned frequency range.
A time division multiple access (TDMA) protocol divides the available transmission time up into discrete time slots and assigns them to individual devices or device pairs. Each device can only transmit in its allotted time, but knows that it won't have a collision with another device.
A code division multiple access (CDMA) protocol allows individual devices to transmit at the same time, but has each device use its own assigned code to encode transmissions. Collision will still occur, but if orthogonal or mostly orthogonal codes are chosen, the effect of such collisions can be minimized.
An ALOHA protocol allows devices to transmit without sensing the channel at all. This results in comparatively higher collision rates and reduced efficiency as more transmissions must be resent. A slotted ALOHA protocol further refines the idea by dividing the available transmission time up into discrete time slots. It uses an ALOHA protocol within those time slots, but only allows each device to start a transmission at the beginning of one of the time slots. Thus, if no collision occurs immediately, it won't happen at all.
A carrier sense multiple access (CSMA) protocol has a potential transmitting device listen to the transmission channel prior to transmitting, and only allows it to transmit if it hears no one else transmitting (i.e., only if the transmission channel is clear). However, this still runs into the danger that two devices will both hear that the transmission channel is clear and start to transmit at the same time, thus causing a collision.
One way to improve CSMA performance is to use collision avoidance (CA). Under a CSMA/CA protocol, each device must first inform all other devices of its intent to transmit. Only once this is done can a device make a data transmission.
Another way to improve CSMA performance is to use collision detection (CD). Under a CSMA/CD protocol, each device monitors the transmission channel prior to transmitting and only starts transmitting if the medium is free, just as with basic CSMA. But the device also continues to monitor for other signals throughout transmission. If it detects another signal and a collision occurs, it stops transmitting and backs off for a time, trying to transmit the data again later. The Ethernet protocol is an example of a CSMA/CD protocol.
However, some low-power, high bandwidth networks (e.g., ultrawide bandwidth networks) may have sufficiently high processing gain (e.g., 10 dB) that a device can successfully send a data transmission even if it overlaps with another transmission. In such a network it may therefore be possible for a device to transmit data even when another transmission is in process. But if the device is in a conventional CSMA protocol (with or without CD), it may nevertheless be forced to avoid transmission while the other device is transmitting. This will unnecessarily limit transmission opportunities and greatly reduce network throughput.
It is therefore desirable to provide a system and method through which a CSMA protocol can be altered to allow for the transmission of multiple simultaneous signals, while still minimizing the chance of fatal collisions.